Conventionally, various image sensing apparatuses using solid-state image sensing elements (e.g., CCD) have been proposed. FIG. 19 is a block diagram showing the arrangement from a solid-state image sensing element to a clamp circuit in a conventional image sensing apparatus. In such an image sensing apparatus, light from an object, which has entered via a lens, is received by a solid-state image sensing element 13, and an output signal from the solid-state image sensing element 13 is input to a CDS unit 14 that performs correlated double sampling (to be abbreviated as CDS hereinafter) to remove clocks and to reduce noise. The output signal from the CDS unit 14 is input to a clamp circuit 15, which clamps the output (corresponds to a black level) from an optical black (to be abbreviated as OB hereinafter) portion, which is formed by light-shielded pixels of the solid-state image sensing element 13, to a given DC potential under the control of a clamp pulse signal CPOB, thereby generating a signal level serving as a black reference. After that, an image signal is A/D-converted by an A/D converter, and the converted image data is input to a signal processor in the subsequent stage to undergo various signal processes and conversions to obtain a signal of a desired display or recording format.
FIGS. 20A and 20B show the basic arrangement of the solid-state image sensing element 13. The solid-state image sensing element has an effective pixel region 111 capable of receiving light from an object, and an optical black region 112 shielded from light by, e.g., aluminum. Pixels in the effective pixel region 111 and optical black region 112 respectively have photodiodes 113 and 114. In FIG. 20B, the photodiodes 113 of the effective pixel region 111 are indicated by open squares, and the photodiodes 114 of the optical black region 112 are indicated by hatched squares. The output from each photodiode 114 is used as a reference for an optical black level.
Furthermore, the solid-state image sensing element has vertical transfer registers 115 arranged along arrays of the photodiodes 113 and 114, and a horizontal transfer register 116 for transferring charge signals from the vertical transfer registers 115 to an output circuit 117. With this arrangement, charge signals photoelectrically converted by the photodiodes 113 are sent to the corresponding vertical transfer registers 115, and are then sequentially sent to the horizontal transfer register 116 in synchronism with vertical transfer pulse signals φV1 to φV4. The charge signals sent to the horizontal transfer register 116 are sequentially sent to the output circuit 117 in synchronism with horizontal transfer pulse signals φH1 and φH2, and are then output to the subsequent circuits. Reference numeral 118 denotes a portion of the horizontal transfer register 116 to which no charge signals are transferred from the vertical transfer registers 115, i.e., a so-called dummy portion.
When a small clamp time constant is set in the clamp circuit 15 of the conventional image sensing apparatus shown in FIG. 19, different OB levels are sampled and held, and clamped between lines, and horizontal stripe-like noise consequently appears on an image, thus influencing image quality. For this reason, in the clamp circuit of the conventional image sensing apparatus, a relatively large time constant upon OB clamping must be set in consideration of image quality and response speed.
However, when very intense light such as spot light, sunlight, or the like has become incident on the solid-state image sensing element 13, charge signals generated by the receivable effective pixel region 111 overflow to reach the vertical transfer registers 115 and horizontal transfer register 116, and many charge signals are transferred even during a charge transfer period of pixels of the OB region 112, i.e., an OB period in which nearly no charge signals are supposed to be transferred. As a result, the OB level of a signal output from the CDS unit 14 becomes higher than a normal level, and is different from the black level to be obtained.
This phenomenon will be examined using FIGS. 21A and 21B. In a normal operation (when no intense light enters), the pixels of the effective pixel region 111 accumulate charges corresponding to object light, and the pixels of the OB region 112 do not produce any charges since they are shielded from light. Hence, the CCD outputs of the pixels of the effective pixel region 111 during the charge transfer period (effective pixel period) and OB period are as shown in FIG. 21A. The CDS unit 14 executes correlated double sampling by sampling and holding a reset level in response to a sample/hold pulse signal SH1, sampling and holding a signal level in response to a sample/hold pulse signal SH2, and extracting their difference as a signal level, thus obtaining a CDS output signal shown in FIG. 21A. Note that the CDS output waveform shown in FIG. 21A is converted to rise upward when the signal level is high (many charge signals are accumulated in the CCDs). When this CDS output signal is clamped during the OB period in which a clamp pulse signal changes to LOW, a constant DC potential is obtained and is used as the reference level upon executing subsequent A/D conversion and various signal processes.
On the other hand, when very intense light such as spot light, sunlight, or the like strikes, and when charge signals of the effective pixel region 111 overflow and are transferred even during the OB period, as indicated by the CCD output in FIG. 21B, the signal level of the CDS output during the OB period becomes higher than the proper black level. In this state, if the clamp circuit 15 clamps the CDS output during the OB period, the higher level is used as the reference level of signals. Hence, the level difference between the reference level and a signal from the effective pixel region 111 becomes small, and a dark video signal is obtained consequently. Furthermore, when the CDS output during the OB period has a higher potential, which becomes equal to that of the CDS output during the effective pixel period, the obtained video signal indicates solid black.
FIG. 22 shows the CDS output waveforms in normal operation during one horizontal period and when charge signals have overflowed into the OB region 112. As in a normal CDS output, if the OB level maintains a proper level, the signal level during the effective pixel period is correctly processed. However, as can be easily understood from the above description, if a signal is clamped at the varied black level indicated by a CDS output obtained when charge signals have overflowed in FIG. 22, a signal equal to or lower than the varied black level is determined to be black, and a dark video signal is obtained. Furthermore, as can be easily understood from the above description, when many charge signals have overflowed during the OB period and the highest levels of CDS output signals obtained during the OB period and effective pixel period have the same potential, a solid black image is consequently obtained.
When incidence of very intense light stops or when exposure is completed by a mechanical shutter like in photographing of a still image, charge signals that overflow into the vertical transfer registers 115 and horizontal transfer register 116 gradually decrease, and also do charge signals read out during the OB period. However, when a relatively large clamp time constant is set for the aforementioned reason, since it maintains a long period of time in which a level different from a proper level is clamped, a considerably long period of time is required until a correct level to be clamped is recovered.
As shown in FIG. 23, when a still image is photographed while charge signals overflow into the vertical transfer registers 115 and horizontal transfer registers 116 corresponding to the OB region 112, the OB level of the CDS output during the exposure period of that still image is higher than a proper black level. After a mechanical shutter is closed and exposure of the still image is completed, since light ceases to enter the solid-state image sensing element 13, there is a period, from the beginning of the read process of the still image from the solid-state image sensing element 13 to a certain timing, in which the clamp level is gradually returning a proper value but a proper black level cannot be obtained. For this reason, a portion of a frame cannot have proper luminance levels and, as a result, a desired image cannot be obtained.